The transfer of ink from gravure cells on a cylinder to the surface of a paper web is enhanced by the application of a DC field between an impression roll and the cylinder, across the web. Ink in a gravure cell has a concave meniscus lying below the cylinder surface. Substantial pressure between the impression roll and cylinder is required for reliable transfer of ink from the cylinder to the web.
It has been found that application of an electrostatic field between the impression roll and cylinder distorts the surface of the ink, causing the ink to extend out of the cell beyond the cylinder surface. Thus, the ink contacts the web in the impression roll-cylinder nip so that the ink transfer is enhanced. George et al. U.S. Pat. No. 4,697,514 shows a method and apparatus for gravure printing with electrostatic assist.
Use of ESA often results in cling, similar to static cling, between the sheets or signatures which are cut and formed from the web. The cling makes it difficult to separate the sheets or to open the signatures causing problems in the bindery or other operations utilizing the printed material.
I have found that the cling is due to the creation of "electrets" in the ink on the web. An electret is a dielectric with a permanent state of electric polarization. When placed in an electric field, dry ink, a dielectric, absorbs electric charges. The absorption results in long-lasting trapped charges in the ink. The ink after absorption produces an electric field. After the electric charge is trapped, surface charge accumulates on the ink. The ink exhibits no external electric field because the surface charge is approximately equal and opposite to the charge in the ink. Eventually, the surface charge dissipates, leaving the charged ink on the web. This electric charge trapped in the ink causes cling.
In accordance with the invention, the potential of the electrets in the ink resulting from use of ESA in a multicylinder press are reduced by operating at least one impression roll with a DC potential of the polarity opposite that of another impression roll. More particularly, the impression roll for the last of multiple printing cylinders has a polarity opposite that of the impression rolls for the preceding cylinders.
Another feature is that the method of reducing potential of the electrets in the ink includes the steps of measuring the potential of the printed web caused by the ink electrets downstream from the last cylinder and impression roll and controlling the DC potential of the last impression roll to minimize the measured potential. Preferably the DC potential is controlled through a feedback circuit.
A further feature of an alternate form of the invention is that an operator observes the measured potential of the printed web while the web is grounded and controls the DC potential of the last impression roll.